Arteriosclerotic coronary artery disease is a common form of heart disease. The disease is caused by a build-up of fibrous and fatty substances on the inner walls of the coronary arteries. The substances combine to form a soft, semi-solid material known as "plaque". Over a period of time the plaque can can become infused with calcium causing it to become hardened. The plaque reduces the size of the lumen in the artery and thereby reduces the blood flow to the heart muscle. If the blood flow is reduced sufficiently, the heart muscle in the area beyond the constriction can be damaged by oxygen starvation resulting in a myocardial infarction, commonly called a heart attack. Alternatively, the plaque build-up may cause turbulence in the blood flow resulting in a thrombosis in the coronary artery, which, in turn, leads to a heart attack.
Direct surgery on the restricted area is often difficult or impossible due to the small size of the affected arteries. Consequently, various procedures have been developed to enlarge atherosclerotic plaque constrictions without requiring incisions in the artery walls, thereby increasing the blood flow to the heart muscle and avoiding heart attacks.
If the stenosis caused by the plaque is not a complete obstruction and if the plaque has a low concentration of calcium (and, therefore, is soft), a dilatation balloon catheter may be used to enlarge the stenosis in the artery. In this conventional procedure, a special catheter with a "balloon" section having expandable walls is inserted into a conveniently located artery and guided through the coronary vasculature to the location of the restriction by means of fluoroscopy and contrast medium injections. The balloon section of the catheter is moved into the constricted area of the artery and pressurized liquid is passed down the catheter to inflate the balloon causing it to expand and compress the plaque against the artery walls. The operation increases the size of the lumen in the artery and therefore increases blood flow.
The dilatation technique, however, does not work where the plaque has a high concentration of calcium and is, therefore, "hard". Also, the technique cannot be used where the plaque has built up to the point where it has formed a complete blockage or where the stenosis is severe and only a very small opening remains, for in these latter situations, the catheter cannot be pushed through the opening.
Accordingly, a number of prior art methods have been devised to open or "recanalize" arteries in areas where the balloon dilatation catheter will not work. In one common method, a catheter is provided with an optical fiber passing along its length. A laser is coupled to the proximal end of the fiber and the laser energy is guided down the fiber and directed onto the plaque to vaporize or melt the plaque. This technique is useful in certain situations, but it suffers from a number of drawbacks.
More particularly, lasers which produce an electromagnetic energy output at a wavelength that is optimally absorbed in the plaque are generally not well-suited to transmission down an optical fiber which requirement is necessary to deliver the laser energy to the area of the constriction. Some lasers are available which do have good tissue absorption characteristics and can be transmitted down an optical fiber, but it is difficult to construct these lasers to generate the required power.
In addition, difficulty is often encountered in constraining the laser energy to the desired area due to scattering. Unless care is used, the interior walls of the artery can be damaged. Such damage can happen, for example, when the laser energy burns through the plaque restriction and then impinges on the artery wall beyond the blockage. For these reasons laser vascular surgery is still in the experimental stages.
As an alternative to laser surgery, another catheter arrangement uses small knives at the distal end to cut away the plaque. This arrangement also suffers from the disadvantages of the balloon dilatation catheter in that it is difficult to use the device in an artery which is tightly constricted.
Accordingly, it is an object of the present invention to provide apparatus for clearing away obstructions in arteries and other blood vessels.
It is another object of the present invention to provide apparatus for clearing obstructions in arteries and other blood vessels which can be used in tightly-constricted blood vessels.
It is a further object of the present invention to provide apparatus which can clear obstructions in arteries and other blood vessels by melting atherosclerotic plaque.
It is yet another object of the present invention to provide apparatus which can remove obstructions in arteries and other blood vessels and which can be advanced along a guidewire that can be easily maneuvered along an artery or other blood vessel.